1. Field of the Invention
The present invention relates to an ultraviolet detector and, in particular, a detector that detects weak ultraviolet radiation emanating from a flame.
2. Related Background Art
An example of such a kind of ultraviolet detectors which have conventionally been used in general employs a configuration shown in FIG. 17. This ultraviolet detector 100 comprises a sealed vessel 101 made of ultraviolet-transparent glass. Within the sealed vessel 101, a planar anode 102 and a planar cathode 103, which oppose each other, are disposed in parallel with each other. The anode 102 is secured to an anode pin 105 penetrating through a stem 104 of the sealed vessel 101, whereas the cathode 103 is secured to a cathode pin 106. Formed between the anode 102 and the cathode 103 is a discharging gap 107 of about 0.4 mm. The voltage between the anode 102 and the cathode 103 is set to a level which is higher than the lowest voltage that induces discharge therebetween in response to incident ultraviolet radiation and at which no spontaneous discharge occurs when there is no incident ultraviolet radiation. A discharged gas is enclosed within the sealed vessel 101.
When a trace amount of ultraviolet radiation emanating from a flame is incident on the sealed vessel 101, the incident ultraviolet radiation passes through a grid-like ultraviolet-transmitting opening 102a formed in the anode 102 and then impinge on the surface of the cathode 103, whereby photoelectrons are emitted from the cathode 103. Thus generated photoelectrons are accelerated toward the anode 102 due to an electric field and collide with molecules of the gas between the anode 102 and the cathode 103, thereby causing an electron avalanche. Due to this electron avalanche, a number of cations are generated between the electrodes 102 and 103. These cations are accelerated toward the cathode 103 by the electric field and collide with the surface of the cathode 103, whereby a number of secondary electrons are emitted therefrom. Like the photoelectrons, the secondary electrons generate electron avalanches, whereby discharge is formed between the electrodes 102 and 103 thereafter. When the current resulting from the discharge is observed, the incidence of ultraviolet radiation, i.e., existence of the flame, is detected.
In the conventional ultraviolet detectors, however, due to the above-mentioned configuration, there have been the following problems.
Namely, since the discharging gap 107 between the anode 102 and the cathode 103 is quite narrow, the sensitivity in detection may fluctuate even when a slight aberration occurs in this gap. In the event that a shock or vibration is imparted to the detector 100 itself, the anode 102 and the cathode 103 may come into contact with each other, thus disabling its normal operation. Here, Japanese Utility Model Publication No. 49-17184 discloses an example of conventional ultraviolet detectors.